1845.] 617 



is scarce in Scandinavia, and as yet has never been discovered in 

 Russia. On the other hand, forms which are rare in England, 

 such as the Illcenus crassicauda (published as the /. perovalis in 

 the Silurian system) and others, become very common both in 

 Scandinavia and Russia. Again, Orthoceratites, usually so scarce 

 in the lower division of the English series, are most prodigiously 

 disseminated through the deposits of the same age both in Scandi- 

 navia and Russia ; while the Orthid<B, with simple ribs, are here as 

 characteristic of the Lower Silurian age as in England, and 

 among them is the Orthis calligramma, which occurs in some of 

 the very oldest strata of N. Wales. 



In the Russian Baltic provinces, as in Scandinavia and England, 

 the Lower Silurian group is terminated in the ascending order by 

 a limestone containing Pentamerus, in which some of the Upper 

 Silurian corals begin to show themselves, but the prevailing 

 species, though closely approaching to the P. oblongus of England 

 and Norway, is one which must be considered a new species, and 

 which we formerly named P. Letticans, from the tract in which we 

 found it.* 



Numerous sections to the south of Czarskoe Celo and on the 

 rivers Ishora, Volkof, Siass, &c. demonstrate that the Lower 

 Silurian group, as clearly defined by its fossils, and without the pre- 

 sence even of the intermediate band of Pentamerus limestone, is at 

 once overlaid by true Devonian strata laden with ichthyolites. 

 Such a junction was formerly described on the river Volkof, but 

 at that period the precise equivalent of the underlying Silurian 

 rock was not pointed out. Li my last visit to St. Petersburg, I 

 visited one of these junctions in company with my friends, Count 

 Keyserling and M. Worth, who had shortly before described it 

 in a memoir read before the Mineralogical Society of that capital. 

 The Pleta or Orthoceratite limestone, which occupies the great 

 plateau of Czarskoe Celo, is surmounted, to the south, at the 

 village of Ontolova, by reddish sandy and marly beds, in which 

 a few fishes' scales were found by M. Worth ; and on following 

 these beds up the course of the Slavenka, to the villages of Ma- 

 rina and Porites, they are seen to become hard cream-coloured 

 marlstones in which the remains of Ichthyolites are most abun- 

 dant, f 



These remains have, in the hands of M. Agassiz, to whom I 

 referred them, thrown much additional light on the Devonian 

 fauna. Among them there are, it is true, certain forms belonging 

 to genera known in the Old Red Sandstone of Scotland and 

 England, but these are accompanied by several genera new to 



* Since our former notices were read it has been published as Pentamerus 

 horealis by Eichwald. 



f In his accurate coup d'oeil of the environs of St. Petersburg, Strangways 

 was the first who had noticed and even marked in his map this red earth on tlie 

 Slavenka ; but he found no fossils in it, and at the period when he wrote could 

 not be expected to compare it with the Old Red Sandstone. 



z z 4 



